Transducer measurement systems are known for measuring various characteristics of an object, such as temperature, thickness, conductivity, etc. In one such system, the transducer provides an output signal that is processed to generate an electrical signal having a voltage level related to the distance between the transducer and the object. In one exemplary use of the system, the transducer is a capacitive probe and the characteristic of the object to be measured is the flatness of the object. Such a measurement is provided by obtaining multiple transducer output signals in response to measurements at multiple locations of a particular object. Examples of typical objects are semiconductor wafers and disks for disk drives.
As is known, such measurement systems generally include a plurality of subsystems, such as inter alia, a transducer subassembly, a "front end" circuit, such as may contain a preamplifier, for converting the transducer output signal into a corresponding electrical signal having a voltage level related to, and ideally proportional to, the measured characteristic, a signal conditioning circuit for conditioning the electrical signal and providing an output measurement signal indicative of the measured object characteristic, and a display circuit for displaying a representation of the measured object characteristic. Ideally, parameters of the constituent subsystems meet predetermined specifications so that the output measurement signal has a known mathematical relationship to the measured object characteristic. However, typically these parameters vary within a range due to such effects as component tolerances and stray capacitance. When the subsystem parameters vary, the output measurement signal may not be an accurate representation of the measured object characteristic, but may be in error in such parameters as gain, offset, linearity, etc.
One technique used for calibrating transducer measurement systems in order to ensure that the output measurement signal maintains a known relationship to the measured characteristic is to perform a "system level calibration" using one or more reference objects (i.e., an object having a known characteristic to be measured). More particularly, output signals from the integrated system, obtained in response to several reference objects, are set to predetermined reference levels by adjusting one or more variable components within the system. That is, the variable component is adjusted until the output signal is brought into conformity with predetermined values. For example, an adjustment may be achieved by adjusting a potentiometer within the system. With this calibration technique, if one of the subsystems is replaced, the system level calibration must be repeated. Thus, obtaining a replacement subsystem requires the user to either send the entire measurement system back to the manufacturer for subsystem replacement and system re-calibration, have a repair person travel to their facility to replace the subsystem and re-calibrate the system, or possess the requisite skill and apparatus for recalibrating the system after installation of a replacement subsystem.
Another technique for calibrating transducer measurement systems is a to perform a "subsystem level calibration" in which each individual subsystem is calibrated separately. More particularly, the output signal from a subsystem, obtained in response to each reference object, is set to a predetermined value for that subsystem by adjusting variable components within that subsystem. Each subsystem is adjusted, until the final output signal is brought into conformity. However, this type of "subsystem level calibration" tends to be labor intensive since adjustments must be made to each individual subsystem.